The present invention relates to a process for manufacturing glass in a furnace in which a glass charge in solid form is introduced into the upstream part of the furnace in a so-called furnace-charging zone in which there is also a zone for removing the combustion smoke from the furnace, the furnace also including a charge-melting zone located in the middle of the furnace and heated by means of at least one burner, as well as a charge-refining zone in which the glass is brought to the desired temperature and viscosity before it leaves the furnace and enters a feed channel of glass-forming machines.
In a glass furnace, the charging of a furnace with glass in the solid state is generally carried out on the upstream part of the furnace while the heating of the charge is more carried out in central part or downstream part of the furnace.
It has been observed that, at least in some furnaces, there were cool spots in the crown zone of the furnace, in particular in the region of the upstream part. These cool spots may be characterized by crown temperatures in this upstream part of less than 1430.degree. C., which causes condensation of alkaline products on the refractories of the crown, which condensation is then responsible for degradation of this crown and contamination of the glass bath by refractory residues resulting from the etching of the wall by these alkaline condensation products.
It is known, for example from EP 532,825 and from EP 508,139, to use flames, in particular oxy-fuel flames, which heat the surface of the glass bath in the central zone, or in the downstream zone of the furnace but oriented towards the upstream zone of the furnace, thus allowing this upstream zone to be heated.
The problem encountered with this type of process is that, when the glass bath is heated significantly, hot spots are produced in the crown of the furnace, substantially in the region of the central zone of the latter (generally the hottest spot). If the energy injected by these burners were to be increased, hoping also to increase the crown temperature in the upstream zone, the maximum temperature permitted by the crown would be exceeded and the refractories in this hot-spot zone would be destroyed. It is therefore necessary to accept a compromise which is not satisfactory as a general rule and which leads to an insufficient temperature in the upstream zone.
The invention makes it possible to avoid these drawbacks. It is characterized in that the energy delivered to the glass in the furnace-charging zone is between 5% and 40% of the total energy delivered to the glass in the furnace and in that the energy delivered to the glass is uniformly distributed over the entire length of the said furnace so as to avoid exceeding a crown temperature at the hot spot of the furnace of greater than approximately 1620.degree. C. and preferably approximately 1590.degree. C., so as to maintain a temperature in the furnace-charging zone, measured in the crown, at least equal to 1430.degree. C. and preferably approximately 1480.degree. C.
Preferably, the energy delivered to the glass in the furnace-charging zone is between approximately 20% and approximately 30% of the total energy delivered to the glass in the furnace according to a preferred embodiment of the invention, in which the furnace includes two smoke-removal ducts located more or less facing each other in the side walls of the furnace near the rear wall of the furnace which includes the port for charging the furnace with glass, the energy being brought into the furnace-charging zone by two burners located on each side of the furnace-charging port and preferably placed symmetrically with respect to the furnace-charging port.
These burners will preferably be so-called flat-flame burners (such as those described, for example, in European Application No. 96 401578.8 or 96 401575.4) preferably having a so-called low-momentum flame (i.e. having a flame fluid velocity of less than 100 m/s and preferably of less than 30 m/s) in order to avoid the fly-off of solid material in the furnace (glass powder) which would then attack the refractory walls of the furnace.
According to a preferred variant of the invention, which will preferably be applied in the construction of new furnaces, the smoke-removal chimneys of the furnace will be placed in a staggered fashion, each in one of the side walls of the furnace near the rear wall of the said furnace where the port for charging the furnace with glass is located, the energy being brought into the furnace-charging zone by means of burners placed more or less opposite each removal chimney.
According to another variant, a furnace configuration may also be provided in which, conversely, the charging of the furnace takes place laterally and at least one smoke-removal chimney is located in the rear wall of the furnace, while still maintaining a burner arrangement identical to that described above.
The invention also relates to glass furnaces allowing the processes described above to be implemented.
The furnace-charging zone as defined in the scope of the present invention is substantially the zone lying upstream of the so-called composition line of the furnace, which is a line well known to those skilled in the glassmaking art.
In general, this composition line lies at the downstream limit (with respect to the directional flow of the glass) of the smoke-removal chimneys (or of the furnace-charging zones in the opposite case).
The invention makes it possible to regularize the temperature profile of the crown of glass furnaces and to increase the productivity of these furnaces by optimizing the transfer of energy to the charge, while at the same time avoiding the drawbacks of the known techniques.
The subject of the invention is also a glass furnace, the wall of which includes, in an upstream end part of the furnace, access zones, namely at least one zone for charging the furnace with a charge to be melted and at least one smoke-removal zone, at least one side of the said upstream end part having at least one of the said access zones, in which furnace at least one region provided with an oxy-fuel burner is located in the said upstream end part, between that one of the said access zones which is furthest upstream and the access zone which is most immediately downstream on one of the sides of the said upstream end part.
By virtue of this arrangement, the cool spots of the crown, which are located over the first meters of the latter in the relevant longitudinal direction of the furnace, going from the upstream end to the downstream end, are eliminated and the temperature profile of the crown is appreciably regularized, that is to say that the temperature of the hot spots is not significantly increased.
The result of this is that the lifetime of the crown is increased, because alkaline condensation has been eliminated, and that the undesirable consequences of fly-off are minimized due to the early vitrification of the charge. In addition, the energy is better distributed over the surface of the glass bath (the specific surface area for heating is increased) and the quality is therefore improved, while the productivity of the furnace is increased.